Process for carbon-coating electrodes



Patented Apr. v 1.9, 1932 ERNEST D. DE BROSKE, OF NORTH BERGEN, JERSEY PROCESS FOR CARBON -COATING ELECTRODES No Drawing.

My invention relates to a process for carhon-coating the electrodes or related parts of electron discharge devices, including the plates or anodcs of radio tubes, to give the coated parts the heat radiation characteristics of a black body.

It is well known that an electron discharge device designed to handle a large amount of power is frequently inefiicient in operation for reasons that are generally ascribed to a secondary emission from the anode to the grid or possibly to the cathode in a direction opposite to the desirable emission of thedevice.

This phenomenon takes place when the anode becomes overheated. T e result is an inter ference between the emission currents sometimes amounting to a complete blocking or choking of the tube. This is particularly the case where the plate is composed of a bright metal such as nickel which is generally used for such purposes. Metals such as this become highly heated by reason of the large amounts of energy consumed by them and be cause of their slight power of radiation of heat. It is well known, of course, that a black body possesses the characteristic of large heat absorption and large heat radiation; and that a nickel anode in a tube of the kind referred to, if blackened, will radiate 3o sufiicient heat to keep its temperature below the point at which secondary emission can take place to any degree suflicient to interfere with the operation of the tube. Accordingly, it has been previously suggested to blacken the anode by coating it with the black oxide or with other black coatings suitable to the metals or the metal used in forming the anode. It has also been proposed and, indeed,-it is the general practice 40 at the present time, to carbonize the plate electrode before mounting it in the tube. However, the methods employed in accomplishing this blackening are expensive and ineficient and result in a large shrinkage and 4 loss during manufacturing operations.

It is the present-day practice to blacken with carbon a large quantity of anode plates in a batch and subsequently to mount the individual blackened plates upon the wire 5 supports provided for the purpose projecting Application filed June 6, 1930. Serial No. 459,609.

from the glass stem of the tube, and thereafter to weld together the plate and the wire supports. In order to blacken the plates, the practice is to place a batch of the loose unmounted plates in a container in a suitable furnace and to heat them to a relatively high temperature of about 850 0., and then to admit a hydrocarbon gas into contact with the hot plates and to continue the flow of said gas over the-plates from ten to thirty minutes. The container with the plates is then pushed into a cooling chamber where the plates can cool in the hydrocarbon atmosphere. The individual blackened plates must then be mounted along with the other 6 electrodes on the stem of the tube, which latter is then assembled with the bulb of the tube and the usual additional steps are then taken to exhaust and finish the tube.

One difficulty with the foregoing presentday practice is that the anodes in the furnace become heated to varying degrees, so that the resulting carbon coating varies accordin to the degree of heat or the position in the urnace in which any vparticular anode may be placed. ,Many such anodes so made have avery slight coating or a grayish coating more or lessadherent, while on others the coating is a mere scale which chips off to such a degree that a second treatment is required. The method also consumes a very large quantity of gas and requires a considerable period of time, ten to thirty minutes and a considerable number of the anodes are rendered unfit for service. Other defects of the present-day practice eliminated by my invention will appear hereafter.

It is an object of the present invention to blacken the anodes or other desired parts after they have been mounted and sealed into the tubes and while they are still on the pump for bombardment and exhaust. It is also an object of the invention to provide a process that consumes a small quantity of gas and which can be accomplished expeditiously. It

is also an object to produce an improved blackened product controllable as to quality and uniformity.

As an illustrat1on of the carrying out of my process I, for example, start with a radio trodes of the tube.

tube made up in accordance with the usual manufacturing method at the stage when it is ready to be exhausted. Its plate, ordinarily made of nickel, is then bright and clean and is welded to the wires which support it v sealed in state in which it is ready for exhausting. I place this sealed-in tube on the automatic exhaust machine and according to usual practice exhaust the air. Then I admit a hydrocarbon gas such as acetylene in a quantity which I prefer shall give a pressure within the tube of less than atmospheric pressure. The hydrocarbon gas having been admitted intothe tube, preferably in measured quantity through a reducing valve, the plate or anode within the sealed-in tube is then heated to a sufficient temperature either by a high frequency induction coil or in any other known manner. The hydrocarbon gas is cracked and the plate becomes black with the deposited carbon within a few seconds. Small plates are satisfactorily coated in this way within about ten seconds.

After the plate is blackened, the tube moves into the next position on the automatic exhaust machine where the excess hydrocarbon gas and the hydrogen and other gases resultmg from the cracking operation are rapidly exhausted from the tube while the plate is still hot, thus preventing undue gas absorption or occlusion.

Instead of acetylene I may use any other suitable gas which will be cracked by the hot electrode with a suflicient deposit of carbon. In case acetylene is used, it is ordinarily only necessary to admit a small quantity into the bulb sufiicient to give a pressure therein less than atmospheric. A pressiire as slight as ten centimeters of mercury or less gives very satisfactory results. The action is very rapid since the acetylene immediately breaks up and deposits a firm adherent carbon coating on the hot anode.

I have even used ordinary illuminating gas to obtain a fine black coatin but the time of treatment had to be extended much longer than that required when acetylene was used.

I may heat the parts to drive out the occluded gases before admitting the hydrocarban gas, but I prefer not to heat the plate until after the hydrocarbon gas hasbeen admitted, in a case Where I want to blacken the plate but not the grid. If I want to blacken also the grid, I can heat the plate for a suflicient length of time before admitting the gas so that the grid also will become heated to the cracking temperature of the gas. Then when I admit the gas, the carbon deposits on both the grid and the plate.

stead any suitable form In case it be desired to blacken the grid to the exclusion of the plate, the grid may be heated by connecting it in an electrical circuit with the filament so that there is sufficient space current between the filament or cathode and the grid to heat the grid to a temperature at which it will crack the hydrocarbon gas and cause it to deposit its carbon thereon.

It will also be understood that these or any other metal parts, including the Wire supports and leads of the electrodes, could be coated with carbon by this process without being assembled as a radio tube, by using inof special container to enclose the parts to be coated and which willpermit of being exhausted, and further will permit of the metal parts being heated in the presence of the hydrocarbon gas in accordance with the process.

It will also be understood that my process does not require the use of an automatic exhaust machine but, on the contrary, can be practised independently of any machine.

In my process the conditions are regulatable so that a coating of the proper depth and quality is uniformly obtainable over the surface of the electrode to the extent desired. This is not possible with the other method of coating the plates in a batch heated to a temperature in a stream of the hydrocarbon gas, because the heating conditions and the extent of contact of the gas with the plates varies depending upon the particular location of the plates be blackened much more than others, and those which are imperfectly blackened will have to be retreated or rejected There are no rejects in my process and substantial uniformity of the coating is obtainable in amount and quality.

To enumerate some of the otheradvantages of my process over the present-day practice :In my process the clean plates or other parts are welded to their wire supports before they are blackened. In this Way perfect welded joints are obtained, whereas there is considerable loss due to defective welded joints when the already blackened plates are sought to be welded to the supports, because the carbon coating prevents metal-to-metal contact and makes welding difficult.

My process also avoids a difiiculty which is inherent in the present-day methods due to situ with no handling being necessary or poshigh' in the batch. Some will aeaaeae I 3 siblg of the plates after they have been'blaekene For a similar reason that the carbon coating is a highly efiicient absorber of gases, my

process, w en practised to make the coating in situ with the excess hydrocarbon and other gases rapidly pumped out while the blackened electrode is still hot, leaves the carbon coating substantially free of occluded gases so that it is a highly sensitive absorbent to take up and hold within itself whatever minute remainder of gaseous or vaporous material there may be left in the tube after it is evacuated. In other words, the carbon 15 coating made by my process is a particularly good getter.

There is also an important economy effected by my process, both in the saving of time and material, over the present-day methods.

My process by comparison uses only a small fraction of the quantity of hydrocarbon gas used by the other method; and, moreover my process by its inherent nature is adapted to be carried on in conjunction with the regu lar process of tube manufacture without any substantial interference or slowing up of said process whatsoever.

It will .be understood that other changw and modifications may be made in the described process illustrative ofmy invention without departing from the scope and spirit of the invention. Having thus described my invention, what I claim is: Q

$5, 1. Process of carbon-coating the plate of an electron-discharge tube which comprises operatively mountin the clean metal plate in the tube, exhausting the tube, admitting a suitable carbon-depositing gas into the tube,

W heating the plate to the carbon-depositing temperature of the gas for a suitable time,- find ire-exhausting the tube while the plate is ot.

2. Process of carbon-coating the plate of 45 an electron-discharge tube which comprises heatingthe clean metal plate while operativelymounted in the tube in the-presence of a suitable carbon-depositing gas to the carbondepositingtemmrature 0 said gas; exhausting the tube while the plateis hot; and finishing and sealin ofi said tube whereby the carbon-coating ecomes agood getter.

* In witness whereof, I have hereunto subscribed my name this 2nd })a% of June, 1930.

55 ERNEST D. BROS. 

